The present invention relates to a spring tester, and in particular to a spring testing device that is capable of providing a load reading at a particular height merely by compressing the test object past the desired check point.
With conventional spring testing devices, in order to obtain a force reading at a desired height, the operator must compress the spring to the desired check point and attempt to maintain the spring at that height for a sufficient period of time to ascertain the correct load reading. As can be readily appreciated, the greater the compression force of the spring, the more difficult it becomes to maintain the spring at the desired compressed height in order to obtain a proper load reading. Furthermore, if a load reading is desired at several different heights, the same procedure must be repeated for each reading. In addition, if the spring rate of a spring is desired, it is necessary to take a reading at a first height, another reading at a second height, and then subtract the two readings. Thus, the inconvenience, the amount of time required, and the potential inaccuracies in obtaining load readings utilizing a conventional spring tester are apparent.
Mechanically operated compressor type spring testing devices are available that employ accurate mechanical stops to eliminate the need for having to manually hold the spring at the desired height setting. However, such devices are substantially more expensive than the manually operated units, and still do not have the capability of obtaining spring rate readings or multiple force readings with a single compression of the test spring.
The present invention provides an improved spring testing device that permits a user to obtain a particular load reading at a preselected height without having to maintain the spring at the desired compressed height. In addition, the present spring tester can provide spring rate readings and multiple force readings with a single compression of the test spring. The spring tester according to the present invention is adapted to provide a load reading at a preselected height merely by compressing the spring past the appropriate check point. The force reading from that setting is automatically retained and displayed on a digital voltmeter or other display device. Thus, the disadvantage of having to manually maintain the spring at the appropriate compressed height is avoided. The present spring tester can also, as noted, provide a spring rate reading with a single compression of the test spring. In particular, by selecting two check points a unit distance apart, and setting the digital display to "rate", the first load reading is utilized to establish the zero reference for the meter so that the reading displayed when the second check point is reached corresponds to the spring rate of the spring.
The present invention includes an electronic control circuit which, in general, comprises a sample and hold circuit that is connected to receive the output from the force transducer mounted in the base of the compressor unit. The output signal from the sample and hold circuit, which is provided to a digital voltmeter, is adapted to follow the signal provided to its input from the load cell. A height indicator, which produces a signal representative of the height of the spring, is connected to a thumbwheel comparator that is adapted to be preset to the height at which the desired load reading is to be taken. When the output signal from the height indicator equals the preselected height, the comparator provides an output signal that is supplied to the sample and hold circuit and is effective to cause the sample and hold circuit to maintain at its output the current load cell reading. Thus, it is apparent that the spring may be compressed past the desired check point and the load reading at the preselected height will be automatically retained.
The control circuit is capable of producing spring rate readings by providing an additional thumbwheel comparator that is connected to a second sample and hold circuit. The output from the second sample and hold circuit is provided to the reference input of the digital voltmeter. Thus, the spring rate of a spring is determined merely by presetting the two comparators to heights a unit distance apart, and compressing the spring through both check points. The output from the second sample and hold circuit provides the zero reference for the voltmeter so that the load reading displayed on the meter when the second check point is passed will correspond to the difference in the load readings at the preselected heights.
Additionally, it will be seen that the present invention can be readily adaptable to provide any number of lead readings at different heights with a single compression of the spring, by adding duplicate circuitry and additional meters. Further objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiments which makes reference to the following set of drawings in which: